The present invention relates to a process for the removal of sulfate ions and, in particular, to a process for the removal of sulfate ions from an electrolyte stream containing halide ions in which they are an interferant or unwanted species.
U.S. Pat. No. 4,485,154 discloses an electrically chargeable anionically active reduction-oxidation system using a sulfide/polysulfide reaction in one half of the cell and an iodine/polyiodide, chlorine/chloride or bromine/bromide reaction in the other half of the cell. The two halves of the cell are separated by a cation exchange membrane.
The overall chemical reaction involved, for example for the bromine/bromide-sulfide/polysulfide system is
Br7+S2xe2x88x92=2Br+Sxe2x80x83xe2x80x83Equation 1
The electrochemical reaction takes place in separate but dependent bromine and sulfur reactions. When discharging, the bromine reaction takes place on the +ve side of the membrane and the sulfur reaction takes place on the xe2x88x92ve side of the membrane. The reaction goes from left to right and metal ions flow from the xe2x88x92ve side of the membrane to the +ve side of the membrane to complete the circuit. When charging occurs the reaction goes from right to left and metal ions flow from the +ve side of the membrane to the xe2x88x92ve side of the membrane to complete the circuit. During extended cycling of the cell anionic species diffuse through the membrane. Thus, although a cation. selective exchange membrane is used, sulfide ions diffuse from the sulphide/polysulfide electrolyte into the bromine/bromide electrolyte where they will be oxidised by the bromine to form sulfate ions according to the reaction scheme below.
HSxe2x88x92+4Br2+4H2O=8Brxe2x88x92+SO42xe2x88x92+9H+xe2x80x83xe2x80x83Equation 2
The occurrence of this reaction is disadvantageous because the presence of sulfate ions in the bromine/bromide electrolyte can result in the precipitation of a sulfate salt. This precipitate can cause scaling within the apparatus, blockage of electrolyte ducts and contamination of the electrodes. In the system described above sodium ions are commonly used as counter-ions and so in this case sodium sulphate is precipitated.
A number of methods are known in the art for the removal of sulfate ions from solution or for the prevention of their precipitation as a salt.
The chlor-alkali industry uses a method known as xe2x80x9cpurgingxe2x80x9d wherein the solution is simply diluted to prevent the sulfate salt from reaching its solubility limit and precipitating out of solution. This method is not suitable in the present process however since the electrolyte is used repeatedly over a large number of cycles whereas in the chlor-alkali processes it may only be recirculated for a few cycles.
Another option is the addition of soluble barium hydroxide and filtration of the resulting barium sulfate precipitate which is highly insoluble in aqueous solutions. This method suffers from the disadvantages that barium salts are very expensive and their toxicity presents environmental problems on disposal. Furthermore, if too much soluble barium is added to the electrolyte it can adversely affect the cation exchange membrane by substituting onto the cation-binding groups (commonly sulfonic acid groups) causing a consequential increase in resistivity, of the membrane. Addition of a soluble calcium salt such as calcium chloride and filtration of the resultant calcium sulfate precipitate is also possible. Calcium salts are less expensive than barium salts and do not cause the environmental hazards associated with the use of barium. However, calcium sulfate is some 1200 times more soluble than barium sulfate, increasing the risk of damage to the membrane and making the process less efficient for reducing the concentration of sulphate. U.S. Pat. No. 4,747,917 discloses a process which includes adding a brine-soluble calcium salt to a brine solution in order to reduce the sulfate ion concentration.
Another possibility for the removal of sulfate is the use of anion exchange resins which selectively remove the sulfate ions. These exchange sulfate ions with, for example, bromide or chloride ions. However, these resins are costly and at present they are not very selective and may remove bromide ions in addition to sulfate ions. They also require periodic regeneration which is a costly process. U.S. Pat. No. 4,556,463 shows the use of an anion exchange medium to remove the sulfate ion prior to returning the solution to the cell.
U.S. Pat. No. 4586993 shows the use of calcium salt addition to form a precipitate of calcium sulfate followed by the use of an ion exchange column as described above.
U.S. Pat. No. 5,587,033 discloses a nanofiltration process for selectively removing multivalent ions from aqueous solution. However, as with ion exchange resins the selectivity is not perfect and considerable amounts of bromide are still lost in the process.
Another possibility for removal of sulfate ions is the crystallisation and separation of a sulfate salt such as sodium sulfate. EP 498919 discloses the use of combinations of refrigeration and crystallisation, and brine recirculation techniques in systems to make brine for electrolysis, which techniques reduce the sulfate ion content of depleted brine. DE 3216418 discloses a process for the cooling and refrigeration of a side-stream of depleted brine so as to crystallise sodium sulfate from the solution. SU 1520012 discloses a process for removing sodium sulfate from brine wherein the brine is subjected to alkali treatment to pH 7.5-9.0 and saturation with NaCl, to bring the ratio of sulfate to chloride to 1:(3-6). Then it is cooled to xe2x88x9220xc2x0 C. and the crystals of sodium sulfate obtained are separated. However, the crystallisation techniques described in the prior art all involve cooling the sulfate contaminated solution which represents a considerable expense.
It is an object of the present invention to provide a method for the removal of sulfate ions from a halogen/halide electrolyte which addresses the problems associated with the prior art.
It is a further object of the present invention to provide a method for the removal of sulfate ions from the halogen/halide electrolyte of a halogen/halide sulfide/polysulfide electrochemical reduction-oxidation system in which the sulfate ions are a contaminant or interferant wit in the halogen/halide electrolyte without interrupting the normal operation of sale system.
Accordingly, the present invention provides a method for the removal of sulfate ions from an electrolyte of an electrochemical reduction-oxidation system wherein said electrolyte comprises a halogen and in which the sulfate ions are a contaminant or interferant, which method comprises the steps of:
(i) increasing the halide concentration in the electrolyte by electrochemical reduction of the halogen,
(ii) crystallising a sulfate salt out of the electrolyte, and
(iii) separating the electrolyte from the crystallised sulfate salt.
Increasing the halide concentration in the electrolyte in step (i) decreases the solubility of the sulfate salt in the electrolyte and therefore promotes the crystallisation of the sulfate salt in step (ii).
The maximum halide concentration attainable will depend on the halogen content of the electrolyte, however, it is preferable that the halide concentration be increased to at least approximately 4M, preferably approximately 5M, and most preferably approximately 6M.
Preferably, before or during step (ii), a seed crystal is added to the electrolyte. In the presence of a seed crystal the sulfate crystallises into relatively large crystals of the sulfate salt which are easier to separate and can be washed free of any bromide contained in the mother liquors. In the absence of a seed crystal, crystallisation does occur but it takes place by spontaneous nucleation which is less desirable and produces smaller crystals which are more difficult to separate. Also, in the absence of a seed crystal, crystallisation tends to occur on the surfaces of the apparatus causing scaling rather than occurring within the body of the electrolyte so as to produce a suspension of crystals in the electrolyte.
Preferably, after step (i), the pH of the electrolyte is adjusted. The solubility of the sulfate salt in an electrolyte containing halide ions is dependent upon the pH and appropriate adjustment results in a more efficient crystallisation process in step (ii). Preferably, the pH is adjusted to a value greater than 1. It is preferable for the pH to be greater than 1 because below this point significant numbers of sulfate ions are protonated to form the more soluble bisulfate ions, thus reducing the efficiency of the crystallisation process in step (ii). More preferably, the pH is increased to a value greater than or equal to 7. It is preferable for the pH to be increased to a value greater than or equal to 7 because this reduces the corrosive nature of the electrolyte and results in less damage to the crystalliser and other equipment which necessarily comes into contact with the electrolyte.
In a preferred embodiment of the process the pH may be increased by continuing electrochemical reduction of the electrolyte obtained after all of the halogen has been reduced to halide ions at a voltage sufficient to cause electrochemical reduction of water in the electrolyte. The electrochemical reduction of water may occur according to the reaction scheme below resulting in the generation of OHxe2x88x92 ions, thus raising the pH of the electrolyte:
2H2O+2exe2x88x92xe2x86x92H2+2OHxe2x88x92
In another embodiment, prior to and/or during step (ii), the electrolyte is cooled to a temperature within the range of from 0 to 20xc2x0 C., preferably from 5 to 15xc2x0 C. and most preferably from 10 to 15xc2x0 C. Cooling the electrolyte further decreases the solubility of the sulfate salt promoting its crystallisation in step (ii). Cooling may be effected by a standard heat exchange apparatus.
Step (ii) is preferably carried out using a standard crystallisation tank.
Step (iii) may be effected by filtration or by the use of a hydrocyclonic separator or a centrifuge.
Preferably, the sulfate salts is crystallised as an alkali metal sulfate. In the case where the sulfate salt is sodium sulfate, crystallisation occurs as the decahydrate, i.e. Na2SO4.10 H2O. This is particularly advantageous in terms of the process water balance. Since water is removed during crystallisation, this allows the crystals to be washed free of residual sodium halide using the water necessary to replenish the process electrolyte. This serves to minimise halide ion loss from the electrolyte. Preferably the water used to wash the crystals is cold, preferably from 0-15xc2x0 C. This minimises the resolvation of sodium sulfate during washing.
In a further embodiment, the present invention also includes a method wherein the sulfate ion concentration of the halogen/halide electrolyte may be monitored. Preferably this is accomplished by means of chromatographic techniques or barium sulfate precipitation measurements.
In a particularly preferred embodiment of the present invention the method is performed on a portion of the total halogen/halide electrolyte volume. This embodiment is preferred because it enables the main system to remain operational whilst the process of sulfate removal is taking place. In this embodiment, step (i) is effected or completed by substantially completing the reduction of the halogen in an external electrochemical reduction-oxidation system so as t0 substantially maximise the halide concentration in said portion of the total halogen/halide electrolyte volume. This embodiment is particularly preferred when the method is applied to electrochemical reduction-oxidation systems which are subject to repeated charge and discharge cycles. This is because, whilst it is desirable that the halide concentration should be maximised in order to minimise sulfate solubility, it is also desirable that the electrolytes within the main system should not be totally discharged because the main system works more efficiently when the charge and discharge cycles are not taken to completion. Also it is desirable that the crystallisation and separation steps be conducted in the absence of any elemental halogen which would cause environmental concerns. In order to ensure complete reduction of the halogen present in the halogen/halide electrolyte it may be necessary to pass it through the external electrochemical reduction-oxidation system a number of times.
Preferably, the amount of halogen/halide electrolyte removed is less than approximately 2% of the total halogen/halide electrolyte volume. More preferably the amount of halogen/halide electrolyte removed is in the range of from 0.1 to 1% of the total halogen/halide electrolyte volume.
The method of the present invention may advantageously be applied to the removal of sulfate ions from the halogen/halide containing electrolyte of a halogen/halide-sulfide/polysulfide electrochemical reduction-oxidation system as described above and in U.S. Pat. No. 4,485,154. In this case step (i) may be effected partially or completely by the normal discharge cycle of said system. During the discharge cycle equation moves from left to, right and consequently the concentration of halide in the halogen/halide electrolyte increases. When the method of removing a portion of halogen/halide electrolyte is applied to the removal of sulfate ions from the halogen/halide containing electrolyte of a halogen/halide-sulfide/polysulfide electrochemical reduction-oxidation system, said halogen/halide electrolyte is preferably removed from the main stream of halogen/halide electrolyte at a time close to the end of a discharge cycle of said system. At this point the halide concentration is already close to its maximum value so less reduction will be necessary in the external cell. Preferably, the portion of halogen/halide electrolyte is removed when the halogen concentration of the electrolyte has been reduced by the discharge cycle of a halogen/halide-sulfide/polysulfide electrochemical reduction-oxidation system to a value in the range of from 0.1 to 0.5M, more preferably to a value of approximately 0.2M.
In the case where the present invention is applied to a halogen/halide-sulfide/polysulfide electrochemical reduction-oxidation system as the main system, an external electrochemical reduction-oxidation cell may advantageously be employed with the halogen/halide electrolyte being supplied to the anolyte chamber of said external cell where it may be electrochemically reduced to increase the halide concentration. The identity of the electrolyte being supplied to the catholyte chamber of said external cell so as to complete the electrochemical reaction is not critical to the invention. However, in a preferred embodiment the electrolyte being supplied to the catholyte chamber may be a sulfide/polysulfide electrolyte. In this. embodiment or sulfide/polysulfide electrolyte from the main system may also be circulated within the external cell, thus removing the need for a separate source of electrolyte for the other half of the external cell.
Alternatively, in another preferred embodiment, the electrolyte being supplied to the catholyte chamber may comprise the halogen/halide electrolyte resulting, from step (iii) of the process of the present invention. That is, the desulfated halogen/halide electrolyte solution is returned to the catholyte chamber of the external electrochemical cell. This results in oxidation of some of the halide back to halogen and therefore partially recharges the halogen/halide electrolyte prior to returning it to the main system.
Alternatively, in another preferred embodiment, the electrolyte being supplied to the catholyte chamber may comprise halogen/halide electrolyte taken directly from the main system. That is, halogen/halide electrolyte solution is drawn from the main system and passed into the catholyte chamber of the external electrochemical cell. This results in oxidation of some of the halide to halogen and therefore partially recharges the halogen/halide electrolyte prior to returning it to the main system. This is particularly advantageous in a halogen/halide-sulfide/polysulfide electrochemical reduction-oxidation system because it helps to rebalance the relative state of charge of the electrolytes circulating within the main system.
The present invention also includes within its scope an electrochemical process for energy storage and/or power delivery comprising the steps of:
(i) maintaining and circulating electrolyte flows in a fully liquid system in which the active constituents are fully soluble in a single. cell or in an array of repeating cell structures, each cell with a chamber (+ve chamber) containing an inert +ve electrode and a chamber (xe2x88x92ve chamber) containing an inert xe2x88x92ve electrode, the chambers being separated from one another by an ion exchange membrane, the electrolyte circulating in the xe2x88x92ve chamber of each cell during power delivery containing a sulfide, and the electrolyte circulating in the +ve chamber during power delivery containing bromine as an oxidising agent,
(ii) restoring or replenishing the electrolytes in the +ve and xe2x88x92ve chambers by circulating the electrolyte from each chamber to storage means comprising a volume of electrolyte greater than the cell volume for extended delivery of power over a longer discharge cycle than the cell volume alone would permit, and
(iii) treating the electrolyte stream containing bromine as an oxidising agent in order to remove sulfate ions from the electrolyte stream.
Preferably, the electrolyte stream containing bromine as an oxidising agent is treated according to the method as defined above in order to remove sulfate ions from the electrolyte stream.